When a two-dimensional signal represented by an image signal is to be stored in a recording medium or transmitted across a network, it is essential to compress or encode the data in order to efficiently use the recording medium or transmission path. Conventionally, various methods such as coding using DCT (Discrete Cosine Transform) and predictive coding have been proposed and used in practice. Recently, information compression methods using wavelet transform are extensively studied, and various proposals are made.
For example, in an encoding/decoding method described in patent reference 1 (Japanese Patent Laid-Open No. 9-148938), a two-dimensional image is first divided into tiles, and wavelet transform is repetitively applied to each tile to hierarchically divide it into frequency subbands. After that, variable-length coding is performed on each subband obtained by the division, and codes are output in order from low-frequency subbands. The merits of the division into subbands are that the code efficiency can be increased by collecting near-by regions having close coefficient statistical properties and performing optimum encoding for each region, and that encoding and decoding can be performed in parallel by hardware. Note that the code efficiency decreases if the division into tiles is excessively performed, so the division size is normally set to about 64×64 or more. However, a method which performs wavelet transform after the division into tiles has the problem that a discontinuous distortion, i.e., block noise is produced in the tile boundary.
Also, in an encoding/decoding method described in patent reference 2 (Japanese Patent Laid-Open No. 2000-316152), each subband having undergone wavelet transform is divided into blocks (8×8), and each block is encoded. This method has the merit that no block noise is produced because the division into blocks is performed after the wavelet transform.
It is possible by performing subband encoding by using the above methods to provide a progressive display function which increases the resolution with time, and reduce the cognitive waiting time of an image viewer. For example, patent reference 3 (Japanese Patent Laid-Open No. 11-298897) has disclosed an image data transmission method by which the size of an image to be displayed increases as the resolution rises.
Also, patent reference 4 (Japanese Patent Laid-Open No. 2003-504921) has disclosed a method which divides an image and draws the divided sub-images at different wavelet decoding rates. More specifically, by using the fact that the central portion of a frame is a region at which the user looks most carefully, details are drawn earliest in sub-images in the central portion and gradually drawn with a delay from the center to the periphery. This display function can also reduce the cognitive waiting time of an image viewer.